Deposition and etching of a layer (film) on a substrate are widely used processes in corrosion protection, antireflection coatings, enhanced surface hardness and wear resistance, as well as microelectronics and other fields of application. For example, microelectronic device manufacturing typically requires deposition of one or more layers on a substrate. In some processes, a layer is deposited on a substrate of the same material to form a homostructure. In other operations, a layer is deposited on a substrate of a different material to form a heterostructure. Often, the layer and the substrate are monocrystalline, such that homoepitaxial deposition and heteroepitaxial deposition are performed.
The layer to be deposited may be one or more single element semiconductors, compound semiconductors, metals, superconductors or dielectrics. Other materials, such as ferroelectric materials may also be deposited. These depositions produce microelectronic devices such as semiconductor integrated circuit chips, optical devices, and thin film magnetic heads. Often, multiple layers are deposited on a substrate to form a superlattice or other structure. Accordingly, a layer may be deposited directly on a substrate, or indirectly on a substrate by deposition on another layer which itself is formed directly or indirectly on a substrate. Similar considerations and terminology apply to etching processes.
Deposition and etching processes require in situ, real-time monitoring of the growth or shrinkage of the layer which is deposited or etched on the substrate. For example, as the integration density of microelectronic devices continues to increase, and new and exotic materials are introduced into microelectronic manufacturing methods, the monitoring and controlling of deposition and etching becomes even more critical, so that the layer has the desired thickness and the desired electrical, optical and other properties.
Many techniques have been developed to monitor and control deposition. For example, Reflection High Energy Electron Diffraction (RHEED) has been developed to monitor the intensity of diffracted electron beams projected across a substrate face. RHEED has allowed atomic layer controllability of initial epitaxial growth.
Some of these techniques involve optical detection because optical detection is nondestructive of the layer being deposited. For example, Surface Photo Absorption (SPA) has been used to monitor the initial growth of a homoepitaxial layer. In this technique, linearly polarized radiation, whose polarization E vector is perpendicular to the substrate surface (i.e. P-polarized) and whose energy (E=h.upsilon.) exceeds the energy gap of the semiconductor being monitored, irradiates a (001)-oriented GaAs substrate at a shallow incidence angle of 70.degree.. This is close to the Brewster's angle. The wavelength is selected to lie in the high absorption region to obtain high surface sensitivity. The change in reflection intensity is explained by the absorption of incident light by the surface chemical species.
Many publications describe the use of SPA to measure initial growth of a homoepitaxial deposition of gallium arsenide on a gallium arsenide substrate. See, for example, Horikoshi, Yoshiji, "Epitaxial Growth of III-V Compound Semiconductor Thin Films and Their Device Applications", Prog. Crystal Growth and Charact., Vol. 23, 1991, pp. 73-126; Kobayashi, Naoki, et al., "Spectral Dependence of Optical Reflection during Flow-Rate Modulation Epitaxy of GaAs by the Surface Photo-Absorption Method", Japanese Journal of Applied Physics, Vol. 29, No. 5, May 1990, pp. L702-L705; Kobayashi, N., et al. "In-situ monitoring of GaAs growth process in MOVPE by surface photo-absorption method", Journal of Crystal Growth, Vol. 107, 1991, pp. 62-67; Horikoshi, Yoshiji, et al., "Optical investigation of GaAs growth process in molecular beam epitaxy and migration-enhanced epitaxy", Journal of Crystal Growth, Vol. 111, 1991, pp. 200-204; Kobayashi, Naoki, et al., "In-situ monitoring and control of atomic layer epitaxy by surface photo-absorption", Thin Solid Films, Vol 225, 1993, pp. 32-39; Kobayashi, Naoki, et al., "Optical Investigation on the Growth Process of GaAs during Migration-Enhanced Epitaxy", Japanese Journal of Applied Physics, Vol. 28, No. 11, November 1989, pp. L1880-L1882; Kobayashi, Naoki, et al., "Investigation of Growth Processes in Flow-Rate Modulation Epitaxy and Atomic Layer Epitaxy by New In-Situ Optical Monitoring Method", citation unknown, 1990, pp. 139-145; Yamauchi, Yoshiharu, et al., "In Situ Spectrum Observation of Ga Deposition Process during GaAs Metal-Organic Chemical Vapor Deposition Using Surface Photo-Absorption", Jpn. J. Appl. Phys., Vol. 32, Part. 2, No. 10A, 1 Oct. 1993, pp. L1380-L1392; Uwai, Kunihiko, et al., "Arsenic Coverages and Surface Structures of As-Stabilized GaAs (001) Surfaces during Metalorganic Chemical Vapor Deposition Observed by Reflectance Difference", Jpn. J. Appl. Phys., Vol. 32, Part 1, No. 12A, December 1993, pp. 5479-5486; Yamauchi, Yoshiharu, et al., "Spectral Observation of As-Stabilized GaAs Surfaces in Metal-Organic Chemical Vapor Deposition Using Surface Photo-Absorption", Jpn. J. Appl. Phys. Vol. 32, Part 1, No. 8, August 1993, pp. 3363-3369; Yamauchi, Yoshiharu, et al., "Decomposition of Arsine and Trimethylarsenic on GaAs Investigated by Surface Photo-Absorption", Japanese Journal of Applied Physics, Vol. 29, No. 8, August 1990, pp. L1353-L1356; Makimoto, Toshiki, et al. , "In Situ Optical Monitoring of the GaAs Growth Process in MOCVD", Japanese Journal of Applied Physics, Vol. 29, No. 2, February 1990, pp. L207-L209; Simko, J. P. , et al., "Surface photo-absorption study of the laser-assisted atomic layer epitaxial growth process of GaAs", Thin Solid Films, Vol. 225, 1993, pp. 40-46; and Zama, Hideaki, et al., "In Situ Monitoring of Optical Reflectance Oscillation in Layer-by-Layer Chemical Vapor Deposition of Oxide Superconductor Films", Jpn. J. Appl. Phys., Vol. 31, Pt. 2, No. 9A, 1992, pp. 1243-1245. See also, Japanese patent application 3-174739 to Kobayashi, et al., published on Jul. 29, 1991.
Coinventor Dietz and H. J. Lewerenz also developed a technique referred to as Brewster's Angle Reflective Spectroscopy (BARS) which is based on the changes of reflectivity of P-polarized light at the substrate Brewster's angle. BARS works at precisely the Brewster's angle of the material being tested. It detects quarter wavelength oscillations in reflected light intensity and thus is theoretically predicted to measure layer thickness. Changes in the reflectivity are large enough to monitor layer growth as well as to reveal the thin film thickness and optical constants of the film. See the publication by Dietz, N., et al. entitled, "An optical in-situ method for layer growth characterization", Applied Surface Science, Vol. 69, 1993, pp. 350-354.
The above-described RHEED and SPA techniques are highly sensitive during the initial atomic monolayers of growth. Unfortunately, RHEED loses sensitivity over the entire duration of the growth process. SPA works at energies where the sample under test is strongly absorbing, thus preventing penetration of the light into the bulk of the film being deposited. Consequently, SPA only probes surface properties. SPA does not allow a correlation of the surface induced signal to the bulk properties of the layer, including its thickness. BARS theoretically predicts that quarter wavelength oscillations in reflected light intensity can measure film thickness, but does not predict other useful monitoring and control information can be obtained from the quarter wavelength oscillations.